The capacity and speed of communications systems may be increased by transmitting information in optical form over networks composed of optically transmissive nodes, fibers, waveguides, and the like. High capacity optical communications systems require that many optical signals be frequency division multiplexed in the components of the optical network. This requires that there be a way of conveniently producing electromagnetic energy at many different frequencies. An ideal device for producing optical energy useful in an optical communications system is a laser, and in particular a multifrequency laser.
Conventional multifrequency lasers (MFL) consists of a waveguide grating router with a shared port on one side comprising one end of a Fabry-Perot cavity, and N array ports containing semiconductor optical amplifiers (SOAs) comprising the other. (See, e.g., M. Zirngibl, C. H. Joyner, L. W. Stulz, U. Koren, M.-D. Chien, M. G. Young, and B. I. Miller, "Digitally tunable laser based on the integration of waveguide grating multiplexer and an optical amplifier," IEEE Photon. Technol. Lett., vol. 6, pp. 516-518, 1994; M. K. Smit and C. van Dam, "Phasar based WDM devices: Principles, design and applications," IEEE J. Select. Topics Quantum Electron., vol. 2, pp. 236-250, 1996; H. Takahashi, S. Suzuki, K. Kato, and I. Nishi, "Arrayed-waveguide grating for wavelength division multi/demultiplexer with nanometer resolution," Electron. Lett., vol. 26, pp. 87-88, 1990; and C. Dragone, "An N.times.N optical multiplexer using a planar arrangement of two star couplers," IEEE Photon. Technol. Lett., vol. 3, pp. 812-815, 1991.
Turning on a particular SOA commences laser oscillation at a particular wavelength as dictated by the router. The conventional MFL thus has N possible oscillation wavelengths. By using N.sub.1 ports with SOAs on one side and N.sub.2 ports with SOAs on the other, one can have N.sub.1 N.sub.2 oscillation wavelength, taking advantage of the N.sub.1.times.N.sub.2 properties of the waveguide grating router (WGR).
In a paper entitled "Arrayed-waveguide grating lasers and their applications to tuning-free wavelength routing," that appeared in IEEE Proc.-Optoelectron., vol. 143, pp. 322-328, 1996, Y. Tachikawa and K. Okamoto demonstrated an N.sub.1.times.N.sub.2 laser (32 channels) using a silica WGR and mechanical switches, but they had to use a large WGR with a free-spectral range of N.sub.1 N.sub.2, had high side modes from unwanted grating orders, and did not achieve single-mode operation. However, as shown by C. R. Doerr and C. H. Joyner, in a paper entitled "Double-chirping of the waveguide grating router," that appeared in IEEE Photon. Technol. Lett., vol. 9., pp. 776-778, 1997, and by C. R. Doerr, M. Shirasaki, and C. H. Joyner, in a paper entitled "Chromatic focal plane displacement in the parabolic chirped waveguide grating router," which appeared in IEEE Photon. Technol. Lett., vol. 9, pp. 625-627, 1997, one can employ chirp in the WGR design to significantly reduce the WGR size and suppress unwanted grating orders, greatly facilitating the achievement of single-mode operation.
Despite advances in the art however, there still remains a continuing need for apparatus that permit one to create a large number of optical frequencies with lasers suitable for optical communications systems.